The state-of-the-art video coding standard, such as H.264/AVC (Advanced Video Coding), H.264/MPEG-4 AVC, and H.265/HEVC (High Efficiency Video Coding) standard, is based on a hybrid coding scheme using block-based prediction and transform coding. To compress a video frame, a video compression system applies prediction methods, such as intra-prediction or inter-prediction, to derive a prediction frame. Based on the prediction frame, the video compression system further encodes residual information to refine the prediction frame. The residual information is usually compressed by transform and quantization techniques. Based on those information, a decoder is able to reconstruct the video frame from the prediction frame and the residual information. The decoder combines a prediction block and a residual block to generate a reconstructed block by simply adding prediction pixel values and residual pixel values to generate final reconstructed pixel values.
A new coding tool, palette mode, is adopted to standardize Screen Content Coding Extensions of High Efficiency Video Coding (HEVC SCC). The palette mode can efficiently describe all pixels in a coding unit (CU) with few selected representative colors. The palette mode is designed based on the observation that the pixel values in a screen content frame are usually concentrated on few color values. An encoder analyzes the pixels in a CU and determines several representative colors to construct a palette table, i.e., a color mapping table between the representative color values and color indices. The palette table is signaled in a bitstream. Pixels with pixel values close to palette colors are quantized to selected palette colors denoted by corresponding palette indices. The rest of the pixels are called escape pixels. A special palette index value is reserved to represent the escape pixels, and pixel values of the escape pixels are signaled directly. All palette indices in the CU form a palette index map, which is transmitted to a decoder along with the escape pixel values. A pixel or a palette index may represent a single color or three-color component values, for example, YCbCr or GBR.
In the HEVC palette mode, if a block is encoded by the palette mode, it will be decoded independently without using any prediction method or residual refinement. To address this issue, a palette-based residual refinement method may be used. However, it is not efficient to apply a full version of the HEVC palette mode or the palette-based residual refinement method on coding units of all sizes when a subset of intra/inter prediction tools is adequate for small coding units.